1. Field of the Invention
The invention refers to an energy storage cell which is used in energy stores, especially in hybrid or electric vehicles.
Hybrid or electric vehicles are vehicles which, in principle, are driven partly or completely by electrical energy.
2. Description of the Related Art
Motor vehicles with a hybrid drive, which are also called hybrid vehicles, have for example an internal combustion engine, an electric machine and one or several electrochemical energy stores. Electric vehicles having fuel cells generally consist of a fuel cell for energy conversion, a tank for liquid or gaseous energy carriers, an electrochemical energy store, and an electric machine for drive purposes.
The electric machine of the hybrid vehicle is, as a rule, designed as a starter/generator and/or an electric drive. In the case of a starter/generator, it replaces the starter and the generator which are normally provided. If designed as an electric drive, the electric machine can provide an additional torque, i.e. an accelerating torque, in order to drive the vehicle forward. As a generator, it enables braking energy and the on-board power network supply to be recuperated.
In a solely electric vehicle, the driving power is provided by an electric machine alone. Both vehicle types, hybrid and electric vehicles, have in common that large amounts of electrical energy have to be provided and transferred.
The flow of energy is controlled by means of electronics which are generally called hybrid controller. It controls, among other things, whether and to which extent energy is to be drawn from or supplied to the energy store.
The energy drawn from the fuel cell or the energy store generally serves to provide driving power and to supply the vehicle's on-board power network. Said energy supply serves to charge the store or to convert braking energy into electrical energy, i.e. regenerative braking.
The energy store for hybrid applications can be recharged while driving. The energy required for this purpose is provided by the internal combustion engine.
The energy suppliers and stores used for electric vehicle applications include for example lead batteries, double-layer capacitors, nickel metal hydride cells or lithium ion cells.
The energy storage cell is accommodated in a gas-tight metal housing in most cases. A special possibility to design lithium ion cells is in the form of a soft pack. This consists of the battery cell which is enclosed by a flexible envelope, typically an aluminium composite film packaging. Due to the geometric similarity with a straight prism, such energy storage cells are also called prismatic energy storage cells.
An important requirement made of these energy stores is to find an optimum of the product of voltage and current for a required power output. This optimization procedure takes into account material and cost aspects. It is found that neither a system designed for high voltages nor one designed for high currents is appropriate for the intended field of application.
Typical voltage ranges for optimum system design are between 100 V and 450 V maximum voltage; the resulting currents can reach 400 A in pulsed operation or even 550 A for special extreme applications and for higher temperature ranges. Continuous currents are in the range of 80-100 A, but they can also be higher, depending on the application. Due to structural and cost reasons, a reduction of these currents in favour of higher voltages involves much more effort than a consistent system design for these high currents.
These requirements do not just apply to energy stores for automotive applications, such as hybrid or electric vehicles, but also in the stationary sector, e.g. for buffering peak loads or in energy stores for decentralized power supply.
Depending on the application as an energy store for hybrid vehicles, plug-in hybrids or as an electric vehicle, maximum power outputs of 10 kW to more than 100 kW are required. Although the requirements made of continuous power outputs can be considerably below these values, such continuous power outputs involve particularly high cooling requirements, especially since the installation space for energy stores is, as a rule, quite limited.
Due to the structure of such high-performance cells (typically >4 Ah), their costs are considerably higher than those of simple consumer cells which, as a rule, also have lower capacities. In addition, the automobile industry requires a service life of more than 10 years.
In order to meet this service life requirement, an efficient cooling concept is indispensable.
WO2007/068223-A1 describes a battery holder with integrated cooling, especially for receiving cylindrical galvanic cells, as they are commonly used in hybrid vehicles, in which battery holder the energy stores are arranged in a honeycomb structure and are cooled by means of two basic heat sinks and at least one intermediate heat sink.
In terms of service life, however, it is not only important that the cell be cooled (absolute cooling), but that it is cooled evenly (relative cooling), i.e. with the lowest possible temperature gradient across the cell and, if several cells are connected to form an energy store, across this energy store, i.e. across the cells. The aim is a temperature difference of ΔT<3 K, ΔT<5 K already being a good value.
Despite an even flow of coolant across or through the entire store, individual cell segments can heat up to a different degree.
In particular those cells which are connected in parallel are not necessarily coupled thermally although they are coupled electrically. Thermal coupling is important, however, to ensure an even flow of current under load. As a rule, it can be assumed that an increase in temperature by 15 K doubles kinetics. Different currents flowing within a parallel circuit in the case of load can accelerate aging of local areas and even cause damage in the case of high currents.